1. Technical Field
The inventive arrangements relate generally to video systems and more particularly to video systems that record or play back digitally encoded video sequences.
2. Description of Related Art
Devices that facilitate the playback of video are gaining popularity in today's consumer electronics marketplace. For example, many consumers have purchased digital video disc (DVD) recorders or players for purposes of viewing previously recorded programs or recording their favorite programs. A DVD recorder or player typically contains a Moving Pictures Expert Group (MPEG) decoder to decode the digitally encoded multimedia data that is stored on the discs that the recorder or player plays. The MPEG video signal to be decoded is comprised of a plurality of groups of pictures (GOP), each of which typically contain an intra (I) picture, a plurality of predictive (P) pictures and a plurality of bidirectional predictive (B) pictures.
During playback of a video signal, some viewers may wish to perform certain trick modes. A trick mode can be any playback of video in which the playback is not done at normal speed or in a forward direction. As an example, a fast-forward trick mode can be initiated to allow the viewer to move through portions of video rather quickly. To effectuate a fast-forward trick mode on an MPEG video signal, the decoder of the DVD may skip a number of pictures in each GOP of the video signal. The faster the trick mode, the greater the number of pictures in each GOP that need to be skipped. Generally, the B pictures are skipped first in successive GOPs until none of them remain, followed by the P pictures until they are exhausted as well. With respect to the P pictures, it is necessary to skip first the P picture at the end of the GOP (this is typically the last picture in display order in a GOP) followed by the immediate prior P picture in display order. This process may continue such that the P picture to be skipped is the last P picture in the GOP (in display order) until no P pictures remain. If desired, the I picture may also be skipped, at which point the entire GOP is skipped.
The principle behind this particular algorithm, in which B pictures are skipped first and P pictures are skipped next in view of their display order, is based on the prediction schemes employed in a typical GOP. Specifically, B pictures are not used to predict other pictures, and it is useful to skip them for a moderate or lower speed-up. In contrast, the I picture is used, both directly and indirectly, to predict all the other pictures in the GOP; if it is the only I picture in the GOP, it must be retained if any of the other pictures in the GOP are not skipped. If the I picture were to be skipped without skipping any of the other pictures, it would be impossible to accurately predict any of the remaining pictures. Similarly, P pictures are used to predict other P pictures and skipping a P picture other than the currently last P picture in the GOP would adversely affect the display of any pictures that follow in display order the skipped P picture.
Although acceptable, the algorithm described above necessitates additional microprocessor programming to conform to the particular order in which pictures are to be skipped. In addition, this skipping algorithm does not permit pictures to be skipped to produce an optimal playback. For example, if a viewer wished to play video back at twice the normal playback speed, the most desirable way to skip pictures in the video would be to skip every other picture. In a typical GOP structure, however, skipping pictures in this manner is unavailable because of the limitations described above.